Polyolefins such as polypropylene and polyethylene are paraffinic hydrocarbons. As such they have no polar groups in their polymer chains. This is advantageous where they are made into fibrous materials such as carpets for their cleanability and stain resistant properties. Their resistance to most acids, alkalis and bleaches however also renders them difficult to dye. They are soluble only in hot chlorinated and aromatic hydrocarbons.
As opposed to polyolefins, most fibers, including polyesters, are dyeable in aqueous mediums. Cotton, wool and nylon fibers, for example, are swollen by water which renders them susceptible to penetration by dye molecules. Under ideal conditions, with a proper match of fiber and dye, most all of the dye in a dye bath is absorbed by the fibers. Once removed from the bath, scoured, rinsed and dried, the dye is held by ionic bonds, covalent bonds, hydrogen bonds and sometimes by physical entrapment. Polyolefins on the other hand, because of their hydrocarbon composition, are not swollen by water. Thus little penetration occurs when they are dyed in an aqueous dye bath, the dye instead being deposited on or near the fiber surface. Not being chemically bonded to the fiber, the dye is rapidly removed by abrasion during normal material use and cleaning.
For these reasons polyolefins have had to be dyed in unconventional manners, primarily by modifying the polymer before dyeing. To this end, metals in the form of inorganic and organometallic compounds have been used for points of attachments for the dyes. A great variety of low molecular weight materials, including salts, alcohols, acids and amines have been added. Although it may seem easy in principle merely to add a dye receptor, this approach has many problems and limitations. For example, the additive must be stable at temperatures up to 600.degree. F. but yet usable. The additive must have at least some compatibility with the polyolefin. It must also be capable of a relatively fine state of dispersion within the fiber. Without dye site uniformity, the fiber itself will not dye uniformly.
In addition, most metal compounds added to polyolefins are dull in shade. Level dyeing has also been very difficult to achieve since once the dye-metal complex is formed, no migration takes place. Thus any slight irregularity of the fiber shows up badly.
Dye sites have also been added to polypropylene by copolymerization with dye site monomers. However this is impractical because of interference by the comonomers in the stereospecific mechanism leading to the formation of the isotactic structure. Most all types of monomers have been used, ranging from chloromethylstyrene to vinylpyridine to vinyl esters. Grafting techniques employing acrylic acids or esters, styrene or alkylstyrenes, vinyl acetate, unsaturated organosilanes, and various halogenated derivatives of these monomers have been tried, but manufacturing cost and thermal instability have hindered their commercialization. Although grafting has the advantage of producing permanent attachment of the dye receptor, it too is both expensive and cumbersome.
A less drastic method of modifying the polyolefin has been to include the additive in the polymer melt before extrusion. The best compatibility with the polymer meld is given by non-polar additives of low molecular weight at relatively high extrusion temperatures. Additives of higher polarity, however, are more effective. Dyeability with disperse dyes has been improved using maleic anhydride, phenols and arylsufonamides.
Partial degradation reactions have also been used to make polypropylene more dyeable. Disadvantages with this approach include deterioration in physical properties, the corrosive nature of many reagents used, and the difficulty of controlling uniformity of the treatment which is essentially a surface treatment. Many different chemical reactions on the polypropylene chain have been attempted. These reactions are based upon the known reaction of aliphatic hydrocarbons with phosphorus trichloride. In the presence of oxygen, the tertiary hydrogen is replaced by a chlorophosphate group. Hydrolysis with water yields a basic-dyeable phosphoric acid. Treatment with an amine or polyamine produces an acid-dyeable phosphonamide site. Other reagents used include sulfuric, nitric, chromic, and chlorosulfonic acids, organophosphorus compounds and silicone halides. As with grafting, the type of modification is usually cumbersome and expensive and thus of little commercial value.